Method of making solar cell

ABSTRACT

A solar cell has a photoelectric conversion semiconductor layer and an electrode made of a material containing a conductive base material and a resin binder electrically connected to the photoelectric conversion semiconductor layer, wherein the volume of voids in the electrode having a diameter of 0.1 μm or greater is 0.04 ml/g or less.

This application is a division of application Ser. No. 07/957,150, filedOct. 7, 1992, U.S. Pat. No. 5,318,638, issued Jun. 7, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solar cell for use as a power supplyfor a variety of electronic equipment or a power source for a powerplant. More particularly, the invention relates to a solar cell havingan improved electrode structure with excellent resistance to theenvironment. Also, it relates to a solar cell which has a cheap,flexible, and durable electrode grid.

2. Related Background Art

In recent years, there has been growing interest in the environment andenergy sources because of global warming and radioactive pollution dueto atomic power plant accidents. In view of these situations, solarcells have been expected to be exploited as a reproducible,inexhaustible source of clean energy. At present, three types of solarcells are well known: single crystal silicon, polycrystalline silicon,and amorphous silicon. The amorphous silicon type solar cell hasexcellent characteristics such as its thin film construction, whichpermits easy realization of large area cells, and a large lightabsorption coefficient, unlike the crystal type solar cells, even thoughit is inferior in conversion efficiency to the crystal type solar cell.Thus, the amorphous silicon type solar cell is one of the more promisingtypes of solar cells. Since its cost is estimated to be significantlyless than the crystal types if production reaches several hundreds ofMW, many studies on it have been performed throughout the world. Anexample of a conventional amorphous silicon type solar cell is shown inFIG. 2. A photoelectric conversion semiconductor layer 103 made ofamorphous silicon is formed on an electrically conductive substrate 104,and a transparent electrically conductive layer 102 also useful as ananti-reflection layer is formed thereon. On the transparent electricallyconductive layer there is formed a grid electrode 101 as a currentcollector. If light 106 is incident on the photoelectric conversionlayer 103 from the grid electrode 101, as shown in FIG. 2, light energyis converted into electric current within the conversion layer 103, andoutputted through the transparent electrically conductive layer 102 viathe grid electrode 101 and the electrically conductive substrate 104.The photoelectric conversion layer 103 contains at least one or more pinor pn junctions, with the p side acting as the anode and the n side asthe cathode.

When the solar cell is used outdoors, particularly good characteristicswith respect to environmental resistance are required. However, studiesby the present inventor have revealed that in a conventional gridelectrode, a short-circuit may take place between the electrodes of thesolar cell due to water permeating voids in the electrode, which is oneof the causes of a decrease in the conversion efficiency. For example, aconventional grid electrode is disclosed in Japanese Laid-Open PatentApplication Nos. 59-167056, 59-167057, and 59-168669. Specifically, asdescribed in Japanese Laid-Open Patent Application No. 59-167056, thegrid electrode is constituted of a conductive paste composed of 80 wt %silver and 20 wt % phenolic resin binder, but it is poor in durabilityand the void volume (as shown at 105 in FIG. 2) may increase with time.With such an electrode, it is difficult to produce a solar cell havingno degradation of conversion efficiency when exposed to the environment.This aspect will be described later in detail.

In general, a solar cell having an output of several W or greater isused outdoors. Therefore, so-called "environment proof" characteristicswith respect to the temperature and humidity are required. Particularlyin a solar cell having a grid electrode as a collector, a conductivematerial such as silver or the like contained in the grid electrode maydissolve by the permeation of water (as shown with Ag₂ O in FIG. 3) andby the photoelectromotive force of the material, diffuse throughdefective portions such as pinholes or exfoliations. This causes ashort-circuit between the positive and negative electrodes of the solarcell, thereby greatly decreasing the conversion efficiency. For example,when the conductive base material is silver, the reaction proceedsbetween the anode and the cathode according to the following formula,thereby giving rise to a short-circuit.

    Anode Ag.sub.2 O and H.sub.2 O→2Ag.sup.+ +2OH       (A)

    Cathode Ag.sup.30 +e43 Ag (dendritic crystal deposition)   (B)

This behavior is shown in FIG. 3. A silver ion 305 arising from apositive-side collector electrode 101 enters a pinhole 306 existing inthe photoelectric conversion semiconductor layer 103 due to an electricfield produced by the absorption of light, and adheres to the conductivesubstrate 104 to form a dendritic type crystal 307. If the dendriticcrystal grows, the collector electrode 101 of the solar cell iselectrically shorted to the conductive substrate 104, so that the outputof the solar cell is reduced. Consequently, degradation of theconversion efficiency may occur.

SUMMARY OF THE INVENTION

In light of the aforementioned drawbacks, an object of the presentinvention is to provide a solar cell which is high in environmentalresistance and unlikely to degrade in conversion efficiency.

The above object of the present invention is accomplished by a solarcell having a photoelectric conversion semiconductor layer and anelectrode made of a material containing a conductive base material and aresin binder electrically connected to the photoelectric conversionsemiconductor layer, wherein the volume of voids in the electrode havinga diameter of 0.1μ or greater is 0.04 ml/g or less.

According to the present invention, water is prevented from enteringvoids existing in the electrode and thus eroding and breaking theelectrode, so that excellent photoelectric conversion efficiency of thesolar cell can be maintained for a long time.

The photoelectric conversion semiconductor layer for use in the presentinvention may be single crystal semiconductor layer, a polycrystalsemiconductor layer, or an amorphous semiconductor layer as a non-singlecrystal semiconductor layer, or a crystalline semiconductor layer.Specific examples include silicon, germanium, carbon, silicon-germanium,silicon carbide, CdSe, CdSeTe, CdTeAs, ZnSe, GaAs or the like. In orderto generate the photoelectromotive force, a pn junction, pin junction,Schottky junction, hetero junction or the like is formed, using such aphotoelectric conversion semiconductor layer.

The electrode for use in the present invention is disposed directly orvia another layer interposed therebetween on at least one face of theabove photoelectric conversion semiconductor layer.

The electrode for use in the present invention will be described below,but to facilitate the understanding of the contents of the art, anexample of the solar cell according to the present invention will bedescribed in which amorphous silicon deposited on a conductive substrateis used as the photoelectric conversion semiconductor layer.

An amorphous silicon layer having at least one pin junction is formed ona flat conductive substrate by plasma CVD using a silane gas. Theconductive substrate may be a plate or sheet of stainless, aluminum,copper, titanium, or a carbon sheet or the like. Also, the substrate maybe a resin substrate having metal or the like deposited thereon.

On such substrate, a transparent conductive layer composed of indiumoxide, tin oxide or the like is formed by vapor deposition, spraying, orthe like. Further, a conductive paste containing a conductive basematerial and a resin binder is applied on the transparent conductivelayer by screen printing or relief printing, and cured at a temperatureof 100° to 200° C., whereby a grid electrode as a current collector isattached. In order to constitute the solar cell of the presentinvention, it is significantly important to select or adjust theconductive paste according to the following criteria found by thepresent inventor as a result of extensive studies.

(1) The amount of solvent contained in the conductive paste should be aslittle as possible.

(2) A resin binder containing the least amount of volatile componentarising in crosslinking of the resin should be used.

The solar cell of the present invention can be constituted by using apaste meeting the above criteria found by the present inventor. In mostcases, the conductive paste contains a solvent such as carhydroacetate,butylcellulobuzo-acetate or polyhydric alcohol, or the like, but thissolvent will increase the void volume in the grid, and is preferablypresent in as little quantity as possible and most preferably is asolventless type paste.

The relation between the solvent weight percent and the void volume isshown in FIG. 6. From FIG. 6, it can be found that the void volumedecreases with decreasing solvent weight percent.

Examples of the conductive base material include silver,silver-palladium alloy, a mixture of silver and carbon, copper, nickel,aluminum, gold, and mixtures thereof. To obtain a conductivity necessaryfor passing electric current through a grid electrode, the conductivebase material is preferably contained at 70 wt %, and more preferably at75 wt % or greater. Preferable examples of the binder resin includeurethanes, epoxies, polyimides, polyesters, phenols, vinyls, acrylics,and the like. Particularly, epoxies are most preferable from theviewpoints of waterproofing and economy. The curing temperature of theconductive paste is preferably as high as possible in order to raise thedensity of crosslinking of the resin, but since too high a temperaturewill burn the resin and thereby increase the gap volume, it should beappropriately selected and set within the range of accomplishing theobjects of the present invention. For example, for urethane or epoxytype resins, it is desirably 200° C. or less. Also, to secure a certaindensity of crosslinking, it is desirable to cure the resin at at least100° C. or greater for more than 10 minutes. Since impurities such aschlorine, sodium, or the like contained in the conductive paste willfacilitate the creation of silver ions as the catalyst in such anelectromigration reaction, it is preferable that it contain the leastamount of impurities possible.

Next, the electrode for use in the present invention and the voidstherein will be described in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic view of a solar cell forexplaining a solar cell according to the present invention.

FIG. 2 is a cross-sectional schematic view of a conventional solar cell.

FIG. 3 is a cross-sectional schematic view for explaining the principleof degradation in the conventional solar cell.

FIG. 4 is a graph showing the result of Example 1.

FIG. 5 is a graph showing the result of Example 2.

FIG. 6 is a view showing the reduction in the void volume withdecreasing weight % of solvent.

FIG. 7 is a view showing the decreasing leakage current increment rateper hour with the reduced void volume.

FIG. 8 is a view showing the improvement in the leakage current withdecreasing chlorine concentration.

FIG. 9 is a view showing the improvement in the leakage current withdecreasing chlorine concentration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a collector electrode 101 is provided on atransparent conductive layer formed on a photoelectric conversionsemiconductor layer 103 which is itself formed on a substrate 104.

Herein, reference numeral 201 indicates relatively large voids having adiameter of, for example, 0.1μ or greater, and reference numeral 202indicates relatively small voids having a diameter of, for example, lessthan 0.1μ.

According to the present inventor, since water flows through the largevoids 201 into the voids 202 where the micro-reaction may occur, it ispossible to substantially prevent water from entering therein if thevolume of the voids 201 is as small as 0.04 ml/g or less.

The shape of the voids is such that the shape of the openings on theelectrode surface and the cross-sectional shape within the electrode maybe any one ranging from a polygon to a circle, an ellipse, a triangle,or a quadrangle. Each void may or may not communicate with one another.

The void diameter and volume as defined in the present invention can beobtained in the following way.

Based on the principle of capillary action, liquid which does notproduce wetting, and thus indicates a contact angle from 90° to 180° tomost substances, will not enter a pore unless force is applied, and willenter the pore in accordance with the applied pressure and the size(diameter) of the pore, so that the volume of mercury permeating thepore can be measured by the diameter of the pore and the work used toforce the mercury into the pore as a function of the applied pressure.With this mercury porosimetry, the diameter and the volume of voids inthe grid electrode can be obtained.

According to the present invention, if the pore or void volume having adiameter of 0.1μ or greater in the electrode is 0.04 ml/g or less, andmore preferably 0.02 ml/g or less, it is difficult for water to reachthe conductive base material, whereby reaction according to thepreviously described formula (A) does not occur, and thus the creationof silver ions is prevented. No short-circuits are formed, so that theconversion efficiency is not degraded.

FIG. 7 shows the relation between the void volume and the leakagecurrent increment rate per hour, which indicates that the leakagecurrent increment rate per hour is advantageously small when the voidvolume is 0.04 ml/g or less.

FIG. 8 shows the leakage current after 10 hours, with the void volumebeing 0.02 ml/g, when a forward bias is applied under the conditions ofhigh temperature and high humidity, and the concentration (density) ofchlorine is changed. FIG. 9 shows an enlarged portion thereof.

From FIGS. 8 and 9, it is indicated that the smaller the concentrationof chlorine contained as impurities in the electrode, the less theleakage of current after 10 hours, and particularly in a range of 0.10wt % or less of chlorine impurity, the leakage current is quite smalland particularly good.

According to the present invention, it is possible to provide a solarcell having a photoelectric conversion semiconductor layer and anelectrode made of a material containing a conductive base material and aresin electrically connected to the photoelectric conversionsemiconductor layer, in which the short-circuit between solar cellelectrodes due to water permeation can be prevented, and theenvironmental resistance is excellent, owing to the facts that:

(1) the volume of voids in the electrode having a diameter of 0.1μ orgreater is 0.04 ml/g or less, or

(2) the volume of voids in the electrode having a diameter of 20 Å orgreater is 0.04 ml/g or less, or

(3) the weight percent of the conductive base material in the gridelectrode is 70% or greater, or

(4) the resin binder is made of urethane, epoxy, or polyimide type, or

(5) the concentration of chlorine contained in the electrode asimpurities is 0.1 wt % or less.

Since a mixture of resin binder and conductive base material is used forthe grid, the grid is attached at a low temperature, so that the solarcell can be fabricated inexpensively.

Since the grid having a resin binder is bendable and durable tomechanical impact, it is possible to provide a flexible and durablesolar cell.

The preferred examples of the present invention will now be described indetail.

EXAMPLE 1

A plurality of photoelectric conversion layers composed of amorphoussilicon were deposited on a stainless substrate having a thickness of 8mils and an area of 16 cm² by plasma CVD, and a transparent conductivelayer composed of indium oxide was adhered thereto by sputtering.

Then, a conductive paste composed of a urethane type resin binder andsilver particulates (70 wt % conductive base material, 18 wt % solvent,12 wt % resin binder) was screen printed, and cured at a temperature of130° C. for one hour to form a solar cell of the present invention. Thetotal volume of voids having a diameter of 20 Å or greater in the gridelectrode measured by mercury porosimetry was 0.036 ml/g, while thevolume of voids having a diameter of 1μ or greater was 0.018 ml/g.

A forward bias of 1.2 V was applied to this solar cell under theconditions of high temperature of 85° C. and a high humidity of 85% RHto measure the leakage current flowing within the solar cell. Theforward bias was applied to simulate the operational state. The leakagecurrent increment rate was 0.2 mA/cm² or less per hour, and it was foundthat elution of silver was greatly decreased. This behavior isgraphically shown in FIG. 4. The ordinate of FIG. 4 indicates theleakage current value per unit area, and the abscissa indicates time.The leakage current increment in this example is indicated by the solidline A. The dashed line indicates the leakage current increment ofComparative Example 1 as next described.

Comparative Example 1

As a comparative example, the same test as above was performed using aconductive paste composed of a polyester type resin binder and silverparticulates (70 wt % conductive base material, 25 wt % solvent, 5 wt %resin binder). The total volume of voids having a diameter of 20 Å orgreater measured in the same way as Example 1 was 0.065 ml/g, while thevolume of voids having a diameter of 0.1μ or greater was 0.046 ml/g. Theleakage current increment rate reached 2.0 mA/cm² per hour.

Comparative Example 2

The same test as in Example 1 was performed using a conductive pastecomposed of a vinyl type resin binder and silver particulates (70 wt %conductive base material, 22 wt % solvent, 8 wt % resin binder). Thetotal volume of voids having a diameter of 20 Å or greater was 0.060cc/g, while the volume of voids having a diameter of 0.1 μm or greaterwas 0.041 cc/g. The leakage current increment rate was 1.8 mA/cm² perhour.

EXAMPLE 2

In the same way, a solar cell was fabricated using a conductive pastecomposed of an epoxy type resin binder and silver particulates (80 wt %conductive base material, solventless). The curing condition was 150° C.for 3 hours. The total volume of voids having a diameter of 20 Å orgreater was 0.018 cc/g, while the volume of voids having a diameter of 1μm or greater was 0.030 cc/g. This is due to the fact that no solvent iscontained and the epoxy type resin binder has a very small amount ofvolatile materials arising during crosslinking. Further, in the same wayas in Example 1, a forward bias was applied to this solar cell under theconditions of a high temperature of 85° C. and a high humidity of 85% RHto measure the leakage current increment. This result is shown in FIG. 5together with that of Comparative Example 1. In FIG. 5, the solid line Aindicates the leakage current in this example, while the dashed line Bindicates the leakage current of Comparative Example 1. In this example,the leakage current increment rate was 0.03 mA/cm² per hour, which wasabout 1/100 of that of the comparative example. It is indicated thatthis value is about 1/10 that of Example 1, and a great effect can beobtained by using such a conductive paste having a small amount ofvolatile material arising during curing.

EXAMPLE 3

Using solventless epoxy type pastes A and B both including 75 wt % of aconductive base material and 25 wt % of a resin binder, with impuritiesmixed in A and impurities removed in B, tandem-type amorphous solarcells A and B were fabricated by the same method as in Example 1. As aresult of measurement by mercury porosimetry, the total volume of voidshaving a diameter of 20 Å or greater was 0.016 cc/g in A and 0.018 cc/gin B. Also, the volume of voids having a diameter of 1 μm or greater wasalmost the same for both A and B, i.e., 0.003 cc/g. The concentration ofchlorine in the electrode was 0.01 wt % in A, but it could not bedetected in B. The measurement method was ion exchange chromatography.The detection limit of the equipment used was 0.001 wt %.

In the same way as in the previous example, a forward bias was appliedto this solar cell under the conditions of high temperature and highhumidity to measure the leakage current. The initial leakage current was0.06 mA/cm² for both. The leakage current after 10 hours was 0.2 mA/cm²in A and 0.07 mA/cm² in B. This indicates that the degradation rate willdepend on the concentration of chlorine.

The present invention can provide a solar cell excellent inenvironmental resistance, wherein the short-circuits between theelectrodes due to water permeation can be prevented by using a gridelectrode in which the volume of voids having a diameter of 0.1 μm orgreater is 0.04 cc/g or less.

Further, the grid can be attached at a low temperature because of theuse of a resin binder and a conductive base material for the grid,whereby the solar cell can be fabricated inexpensively.

Further, since the grid having the resin binder is bendable and durableto mechanical impact, it is possible to provide a flexible and durablesolar cell.

As above described, according to the present invention, it is possibleto provide a solar cell excellent in environmental resistance at a lowercost, so that its industrial value is quite large.

Although the present invention has been described with reference to thespecific examples, it should be understood that various modificationsand variations can be easily made by those skilled in the art withoutdeparting from the spirit of the invention. Accordingly, the foregoingdisclosure should be interpreted as illustrative only and is not to beinterpreted in a limiting sense. The present invention is limited onlyby the scope of the following claims.

What is claimed is:
 1. A method of fabricating a solar cell comprising aphotoelectric conversion semiconductor and an electrode comprising aconductive base substance and a resin binder, said electrodeelectrically connected to said photoelectric conversion semiconductor,said method comprising the steps of:forming a conductive pastecomprising said conductive base substance and said resin binder in whichthe amount of solvent contained therein is adjusted so as to be not morethan 18 wt % based on the weight of the conductive paste, whereby thevolume of voids in said electrode having a diameter of 0.1 μm or greateris 0.04 cc/g or less; applying said conductive paste to a surface ofsaid photoelectric conversion semiconductor; and forming said electrodeby curing said conductive paste.
 2. The method according to claim 1,wherein said semiconductor is a non-single crystalline semiconductor. 3.The method according to claim 1, wherein chlorine is contained in saidelectrode in amounts of 0.1 wt. % or less.
 4. The method according toclaim 1, wherein in the electrode forming step, said conductive paste isthermally cured.
 5. The method according to claim 4, wherein thetemperature of said thermal curing is from 100° C. to 200° C.